Here is what I did to renew/restore the manifold for my 57. I used two products made by the company Por 15. First was "Fire Seal 2000" which can withstand temps up to, you guessed it, 2000 degrees. This product is intended to fill pitting and cracks in manifolds and exhaust systems. The second was "Por 20" which is another high heat treatment that has an aluminum finish. The pics tell the rest:

Here is the manifold still mounted. Plenty of rust and serious pitting.

Manifold after being sandblasted.

Manifold after grinding out all of the peaks and imperfections. I used a dremel with a grinding attachment and then sandblasted it again.